In the development of internal combustion engines, it is a basic aim to minimize fuel consumption, wherein the emphasis in the efforts being made is on obtaining an improved overall efficiency. Fuel consumption and thus efficiency pose a problem in particular in the case of Otto-cycle engines, that is to say in the case of a spark ignition internal combustion engine. The reason for this lies in the principle of the operating process of the Otto-cycle engine.
The Otto-cycle engine operates—if direct injection is not provided—with a homogeneous fuel/air mixture which is prepared by external mixture formation by virtue of fuel being introduced into the inducted air in the intake tract. Load control is generally carried out by means of a throttle flap provided in the intake tract. The further the throttle flap is closed the higher the pressure loss of the inducted air across the throttle flap, and the lower the pressure of the inducted air downstream of the throttle flap and upstream of the inlet into the at least two cylinders. For a constant combustion chamber volume, it is possible in this way for the air mass, that is to say the quantity, to be set by means of the pressure of the inducted air. This also explains why said type of quantity regulation has proven to be disadvantageous specifically in the part-load range, because low loads uses a high degree of throttling and a significant pressure reduction in the intake tract, as a result of which the charge exchange losses increase with decreasing load and increasing throttling.
To reduce the described losses, various strategies for dethrottling an internal combustion engine have been developed. Owing to the fact that the Otto-cycle engine exhibits poor efficiency in part-load operation as a result of throttling, but by contrast the diesel engine exhibits greater efficiency, that is to say lower fuel consumption, owing to the quality regulation, tests have been carried out with regard to combining the two working processes with one another in order to make it possible to apply the advantages of the diesel engine process to the Otto-cycle engine process.
The conventional Otto-cycle engine process is characterized by mixture compression, a homogeneous mixture, spark ignition and quantity regulation, whereas the traditional diesel engine process is characterized by air compression, an inhomogeneous mixture, auto-ignition and quality regulation. One approach to a solution for dethrottling the Otto-cycle engine is for example an Otto-cycle engine working process with direct injection. The direct injection of the fuel is a suitable means for realizing a stratified combustion chamber charge. The mixture formation takes place by the direct injection of the fuel into the cylinder or into the air situated in the cylinders, and not by external mixture formation, in which the fuel is introduced into the intake air in the intake tract.
Another option for optimizing the combustion process of an Otto-cycle engine may be the use of an at least partially variable valve drive. By contrast to conventional valve drives, in which both the lift of the valves and also the timing are invariable, these parameters which have an influence on the combustion process, and thus on fuel consumption, can be varied to a greater or lesser extent by means of variable valve drives. The ideal solution would be fully variable valve control which permits specially adapted values for the lift and the timing for any desired operating point of the Otto-cycle engine. Noticeable fuel savings can however be obtained with partially variable valve drives. Throttling-free and thus loss-free load control is already possible if the closing time of the inlet valve and the inlet valve lift can be varied. The mixture mass which flows into the combustion chamber during the intake process is then controlled not by means of a throttle flap but rather by means of the inlet valve lift and the opening duration of the inlet valve.
A further approach to a solution for dethrottling an engine is offered by cylinder deactivation, that is to say the deactivation of individual cylinders in certain load ranges. The efficiency of the engine in part-load operation can be improved, that is to say increased, by means of a partial deactivation because the deactivation of one cylinder of a multi-cylinder internal combustion engine increases the load on the other cylinders, which remain in operation, if the engine power remains constant, such that the throttle flap can or may be opened further in order to introduce a greater air mass into said cylinders, whereby dethrottling of the internal combustion engine is attained overall. Furthermore, during the partial deactivation, that is to say at part load, the cylinders which are permanently in operation often operate in the region of higher loads, at which the specific fuel consumption is lower. The load collective is shifted toward higher loads.
The cylinders which remain in operation during the partial deactivation furthermore exhibit improved mixture formation owing to the greater air mass supplied, and tolerate higher exhaust-gas recirculation rates. Further advantages with regard to efficiency are attained in that a deactivated cylinder, owing to the absence of combustion, does not generate any wall heat losses owing to heat transfer from the combustion gases to the combustion chamber walls.
Disclosed herein is a system and method for a variable displacement engine which addresses the above described problems and improves on conventional engines with deactivatable cylinders. The object of the present disclosure is an internal combustion engine having at least two cylinders, forming at least two groups. The cylinders of a first group remain active during partial loads, the cylinders of a second group are deactivated when a predefined load is undershot. The at least two groups are characterized by different compression ratios εi, the at least one cylinder of a first group having a greater compression ratio ε1 and the at least one cylinder of a second group having a lesser compression ratio ε2, where ε2<ε1, the at least one cylinder of the second group comprises an activatable and deactivatable cylinder.
In part-load operation of the internal combustion engine, the at least one cylinder of the second group is deactivated if a predefined load is undershot, whereby the load demand on the at least one remaining cylinder increases, and an opening of the throttle flap may introduce a greater air mass into said cylinder.
In addition to said effect which contributes to the dethrottling of the internal combustion engine, the partial deactivation in part-load operation is further optimized by means of a structural feature of the internal combustion engine according to the disclosure, specifically by virtue of the fact that, in the internal combustion engine according to the disclosure, the group of cylinders permanently in operation and the group of activatable and deactivatable cylinders have different compression ratios εi.
In the present case, the cylinders, which are permanently in operation, of the first group have a higher compression ratio ε1, such that in part-load operation, said cylinders have a noticeably higher efficiency η, in particular a higher efficiency than if said cylinders had the lower compression ratio ε2 of the activatable and deactivatable cylinders. It must be taken into consideration here that the efficiency η correlates more or less with the compression ratio εi, that is to say the efficiency η is generally higher in the case of a relatively high compression ratio εi and is generally lower in the case of a relatively low compression ratio εi.
The different compression ratios εi are the result of the configuration of the cylinder groups for different operating or load ranges. Whereas the cylinders which are permanently in operation are configured for part-load operation of the internal combustion engine, the group of activatable cylinders is configured for higher, high and maximum loads. The cylinders, which are permanently in operation, of the first group may be provided with a higher compression ratio ε1 and thus configured for optimized part-load operation of the internal combustion engine, because an increased knocking tendency is not to be expected in said load range. By contrast, the activatable cylinders, that is to say the cylinders which are activated in the case of increasing load demand, are configured for high loads. According to the disclosure, said cylinders should be provided with a relatively low compression ratio, because the knocking tendency increases not only with the compression ratio but rather also with increasing load. Although this reduces the efficiency of said cylinder group, it provides security against knocking at high loads at which said cylinders are primarily used.
A system is provided for an internal combustion engine comprising, at least two cylinders wherein the at least two cylinders form at least two groups, wherein each group comprises at least one cylinder, the at least one cylinder of at least one group being formed as a cylinder which can be activated in a load-dependent manner and which is deactivated if a predefined load is undershot. The at least two groups are characterized by different compression ratios, the at least one cylinder of a first group having a higher compression ratio and the at least one cylinder of a second group having a lower compression ratio and the at least one cylinder of the second group comprises an activatable cylinder. Use of the first cylinder group, and deactivation of the second cylinder group during partial loads increases engine efficiency and fuel economy.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Further, the inventors herein have recognized the disadvantages noted herein, and do not admit them as known.